Here's a more detailed solution based on Jianingy answer. It fixes a few issues I was having after upgrading to Ubuntu 14.10. The issues it's fixing include:

• Multimedia keys not working


• Display settings in Gnome-control-center not working

You need to add DesktopName=Unity to /usr/share/gnome-session/sessions/xmonad.session.

[GNOME Session]

Name=Xmonad/GNOME

RequiredComponents=gnome-settings-daemon;gnome-panel;xmonad

DesktopName=Unity

Also, regarding dbus-send changes in Ubuntu 14.10, you have to use --print-reply=literal instead of --print-reply=string. I have added that to my xmonad.hs and I define main as main = xmonad $ gnomeConfig2.

import XMonad.Config.Desktop

import XMonad.Util.Run (safeSpawn)



import qualified Data.Map as M



import System.Environment (getEnvironment)



data RescreenToggleState = RescreenToggleState Bool deriving Typeable

instance ExtensionClass RescreenToggleState where

  initialValue = RescreenToggleState True



gnomeConfig2 = desktopConfig

    { terminal = "gnome-terminal"

    , keys     = gnomeKeys <+> keys desktopConfig

    , startupHook = gnomeRegister2 >> startupHook desktopConfig }



gnomeKeys (XConfig {modMask = modm}) = M.fromList $

    [ ((modm, xK_p), gnomeRun)

    , ((modm .|. shiftMask, xK_q), spawn "gnome-session-save --kill") ]



-- | Register xmonad with gnome. 'dbus-send' must be in the $PATH with which

-- xmonad is started.

--

-- This action reduces a delay on startup only only if you have configured

-- gnome-session>=2.26: to start xmonad with a command as such:

--

-- > gconftool-2 -s /desktop/gnome/session/required_components/windowmanager xmonad --type string

gnomeRegister2 :: MonadIO m => m ()

gnomeRegister2 = io $ do

    x <- lookup "DESKTOP_AUTOSTART_ID" `fmap` getEnvironment

    whenJust x $ sessionId -> safeSpawn "dbus-send"

            ["--session"

            ,"--print-reply=literal"

            ,"--dest=org.gnome.SessionManager"

            ,"/org/gnome/SessionManager"

            ,"org.gnome.SessionManager.RegisterClient"

            ,"string:xmonad"

            ,"string:"++sessionId]